In dry climates with clear skies and moderate night temperatures, night-roof-spray (NRS) cooling systems (see U.S. Pat. No. 5,174,128 (Bourne) and U.S. Pat. No. 5,542,260 (Bourne)) provide a less-energy intensive alternative to vapor-compression technology for cooling thermal storage water, but this opportunity is relatively unknown in the industry, due to inadequate marketing and general fear of water on the roof. In dry conditions, NRS cooling offers an attractive combination of higher efficiency compared to evaporative and vapor-compression cooling systems, and lower water use compared to evaporative systems, since approximately 50% of NRS heat rejection is by radiation to the night sky; this heat transfer mode uses no water. Ice-making thermal storage systems need less storage volume but cannot take advantage of the NRS opportunity, and they lose efficiency due to low refrigerant evaporating temperatures. Cool water is a near-ideal thermal storage medium, since it doubles as a heat transfer fluid and is inexpensive to contain if space is available. Phase-change thermal storage materials that “freeze” at 55-60 F could use NRS cooling and are therefore attractive for the future, but so far they have high first costs and poor reliability.
NRS systems that collect spray-cooled water on the roof surface and store it to cool the building can also collect rainwater. NRS systems do use water, but if the storage reservoir is sufficiently large, they can operate through a typical cooling season just on collected rainwater. Evaporative cooling systems use at least twice as much water over a cooling season, so it is less practical to cool evaporatively without using piped-in “makeup” water. Also, in many dry-climate regions, water used in buildings comes from ground wells. Groundwater is usually laden with hardness minerals that cause maintenance problems in both water service systems and evaporative cooling systems. Since NRS systems typically use only 2″ to 4″ of rooftop water per year and most populated dry climate areas experience 8-30″ of annual rainfall, it can be practical to implement water storage systems that satisfy full season cooling requirements without adding or purchasing water from a refill source. This strategy has the added advantage that rainwater contains no hardness minerals, minimizing maintenance issues in well-designed NRS systems.
For washing solar arrays, the prior art shows several “once-through” designs such as shown in U.S. Pat. No. 4,119,863 (Kelly), with nozzles for washing a solar array, and in a more recent European patent EU 4,119,863 (Magri), an invention that ambitiously advances a windshield-wiper-style array-washing mechanism. Aside from patents, several solar array-washing systems are commercially available. But none uses an “every night” recirculating water system. One advertised product includes soap dispensing (http://www.solarpanelcleaningsystems.com/). This and the wiper-design strategy apparently assume that washing every night will use too much water, and so they wash infrequently, thus requiring soap and even a wiping mechanism to remove the baked-on dirt. By comparison, the integrated approach of this invention uses the NRS feature justified for its cooling function and extends it to array cleaning as well. There is no added water use for this extension; and the wash frequency prevents dirt layers from “baking on,” eliminating the need for wipers and soap. Also, the use of filtered rainwater assures effective cleaning without residue.